This invention relate to a ceramic composition, especially suitable for use as a dielectric material for fabricating a laminated capacitor, having a high dielectric constant, a high specific resistivity at temperatures of 20.degree. C. and 125.degree. C., a high mechanical strength and capability of being sintered at a temperature of from 900.degree. to 1050.degree. C.
When manufacturing a practical laminated capacitor by using a ceramic composition, as its electric characteristics, such factors as dielectric constant, percentage of variation thereof due to temperature variation, specific resistivity, dielectric loss, and DC bias dependency, etc. must be evaluated. More particularly, the dielectric constant and the specific resistivity must be high as far as possible, whereas the percentage of variation of the dielectric constant due to temperature variation, dielectric loss and variation in the dielectric constant caused by the DC bias must be small as far as possible. Regarding the specific resistivity, according to EIA specification Class II, it is specified that the insulating resistance should be higher than 7,500 Meg ohms or the product of resistance and capacitance (RC product) should be higher than 75 ohm F(=75 meg ohm.multidot..mu.F).
More particularly, unless the product of the dielectric constant and specific resistivity of a ceramic composition is larger than a predetermined absolute value, it is impossible to manufacture a capacitor having any capacitance, especially a large capacitance, and satisfying a practical regulation. Accordingly, the field of use of a ceramic composition not satisfying this condition is greatly limited. In an ordinary laminated capacitor, (n+1) internal electrodes and n (where n is a positive integer) capacitor layers having the same thickness are alternately laminated. Denoting the capacitance of each layer by Co, and the insulating resistance thereof by Ro, the capacitance C of the laminated capacitor becomes nCo, while insulating resistance R becomes Ro/n. Denoting the dielectric constant of the ceramic composition by .epsilon., the dielectric constant of vacuum by .epsilon.o, the specific resistivity of the ceramic composition by .rho., the thickness of each layer by d, and the overlapping electrode area by S, the capacitance Co of a single layer capacitor becomes .epsilon.o.epsilon.S/d, and Ro becomes .rho.d/S. Consequently, the product C.times.R of the capacitance C of an n-layer laminated capacitor and the insulating resistance R is expressed by an equation, EQU (.rho.D/nS).times.(n.epsilon.o.epsilon.S/d)=.epsilon.o.epsilon..rho..
Thus, the CR of the laminated capacitor having any capacitance becomes constant or normalized, i.e., .epsilon.o.epsilon..rho..
A requirement that RC is to be higher than 75 ohm F can be fullfilled when RC=.epsilon.o.epsilon..rho.=8.855.times.10.sup.-14 (F/cm).times..epsilon..times..rho..gtoreq.75 ohm.multidot.F and accordingly, when .epsilon..rho..gtoreq.8.47.times.10.sup.14 ohm.multidot.cm. For example where .epsilon.=5000, a relation .rho..gtoreq.1.69.times.10.sup.11 ohm.multidot.cm is required, and where .epsilon.=10,000, a relation .rho..gtoreq.8.47.times.10.sup.10 ohm.multidot.cm is required. With any large capacitance, laminated capacitors formed of ceramic compositions having a value of .rho. larger than these values in accordance with the dielectric constant can satisfy a condition RC.gtoreq.75 ohm.multidot.F. Where .epsilon. is 5,000 and .rho. is less than 1.69.times.10.sup.11 ohm.multidot.cm, to obtain an insulating resistance higher than 7500 meg.multidot.ohm, the upper limit of the capacitance would be 0.01 .mu.F. As a consequence, with a ceramic composition having a low specific resistivity, it is impossible to obtain a laminated capacitor having a small size and a large capacity, the features of the laminated capacitor.
Where a stringent reliability is required, the BX characteristic defined by U.S. Military Specification (MIL), for example, requires an insulating resistance of more than 100,000 meg.multidot.ohm at 25.degree. C. or a product RC of more than 1000 meg.multidot.ohm.multidot..mu.F. It is also defined that at 125.degree. C. the insulating resistance should be 10000 meg.multidot.ohm or more or the product RC should be 100 meg.multidot.ohm.multidot..mu.F or more.
A high insulating resistance, or a small leakage current not only at room temperature but also at a high temperature, ensures that dielectric breakdown caused by thermal runaway due to an increased leakage current can be prevented and that the reliability of the capacitor can be improved.
In a laminated chip capacitor wherein the layers of the chip capacitor are soldered on a substrate, owing to the difference in their thermal expansion coefficients, the chip is subjected to a mechanical stress and tends to be cracked or damaged. In the case of a dip capacitor encapusulated in an epoxy resin casing or the like, the dip capacitor sometimes cracks due to stress applied by the encapusulation. In any case, where the ceramic composition comprising the laminated capacitor has a low bend strength, the tendency to cracking and breakage increases, thus decreasing reliability. For this reason, it is practically important to increase as far as possible the bend strength of the ceramic composition.
In addition, the internal electrodes utilized in laminated capacitors is required to have a melting point higher than the sintering temperature of the ceramic composition. For this reason, in order to make possible to use inexpensive Ag (silver) electrodes, it has been necessary to decrease as far as possible the sintering temperature. Accordingly, development of a new material that can substitute for BaTiO.sub.3 system ceramic sintered at a temperature of from 1300.degree. to 1400.degree. C. has been desired. For example, U.S. Pat. No. 4,048,546 to R. J. Bouchard et al discloses a Sr.sub.x Pb.sub.1-x TiO.sub.3 -Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 type ceramic. In this reference, .epsilon. is at most 8050 but nothing is taught about the specific resistivity and the mechanical characteristics of the ceramic. Japanese Preliminary Patent Publication No. 111011/1980 by Fujiwara et al discloses Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 -PbTiO.sub.3 -Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 type ceramic. This reference does not at all teach the specific resistivity and the mechanical characteristics. Japanese Preliminary Patent Publication No. 144610/1980 by Fujiwara et al teaches addition of Pb(Mn.sub.1/2 W.sub.1/2)O.sub.3 to Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 -PbTiO.sub.3 -Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 for the purpose of improving the insulating resistance. As a reference, in Table 1 of this publication is shown the insulating resistance of a ternary composition Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 -PbTiO.sub.3 - Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3. Although the insulating resistance is not converted into specific resistivity, a material constant, the specific resistivity can be expressed as follows on the assumption that the percentage of shrinkage is 15% (an ordinary value) and by taking into consideration dimensions after molding. Then the insulating resistance 7.times.10.sup.9 ohms of the composition No. 6 [Pb(Mg.sub.1/3 Nb.sub.2/3).sub.0.45 Ti.sub.0.25 (Mg.sub.1/2 W.sub.1/2).sub.0.30 O.sub.3 ] shown in the table 1 is shown by 2.1.times. 10.sup.11 ohm.multidot.cm in terms of the specific resistivity. Consequently, EQU .epsilon..epsilon.o.rho.=108 meg.multidot.ohm.multidot..mu.F, where .epsilon.=5760.
In the same manner, the insulating resistance, 8.times.10.sup.9 ohm, of sample No. 11 having a composition [Pb(Mg.sub.1/3 Nb.sub.2/3).sub.0.15 Ti.sub.0.30 (Mg.sub.1/2 W.sub.1/2).sub.0.55 O.sub.3 ] is shown by 2.4.times.10.sup.11 ohm.multidot.cm in terms of the specific resistivity. Thus, .epsilon..epsilon.o.rho.=98 meg.multidot.ohm.multidot..mu.F (where .epsilon.=4560). The insulating resistance, 2.times.10.sup.10 ohm, of sample No. 16 having a composition [Pb(Mg.sub.1/3 Nb.sub.2/3).sub.0.15 Ti.sub.0.60 (Mg.sub.1/2 W.sub.1/2).sub.0.25 O.sub.3 ] is shown by 6.1.times.10.sup.11 ohm.multidot.cm in terms of the specific resistivity. Thus, .epsilon..epsilon.o.rho.=209 meg.multidot.ohm.multidot..mu.F (where .epsilon.=3900). Even when the percentage of shrinkage varies by 15.+-.5%, the specific resistivity varies by less than .+-.10% of the values shown just above. In other words, the specific resistivity of the [Pb(Mg.sub.1/3 Nb.sub.2/3 )O.sub.3 -PbTiO.sub.3 -Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 ] type composition is not sufficiently high. Japanese Preliminary Publication No. 144610/1980, Fujiwara et al, contemplates an improvement in the insulating resistance of Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 -PbTiO.sub.3 -Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 but in this Publication, any composition having a dielectric constant larger than 10,000 is not disclosed.
As described above, a variety of known ceramic compositions are required to have higher specific resistivity and dielectric constant at temperatures of 20.degree. C. and 125.degree. C.